[Backend Tester] Pick backend test changes onto release/1.0

[GregoryComer]

Summary

This is a manual cherry-pick of backend testing PRs onto release/1.0. As there are a sizable number of key PRs, I've pulled in all changes under backends/test/suite via git checkout upstream/main -- backends/test/suite/ and then manually picked the relevant files under .github/workflows and .ci/scripts. This should be low-risk, as it only affects backend test logic.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14553

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❌ 3 New Failures, 2 Cancelled Jobs, 2 Unrelated Failures

As of commit a2d30a7 with merge base e0dda90 ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-wheel-packages-linux (3.10) / linux-job (gh)
  RuntimeError: Command docker exec -t 5c4d582bbe1098210c051f4eb35b9545c1c93bd07d850f47e6b489eefbb31686 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t e086134aeb1a52d5290703c26262428c682241002c28fc15db33b0276682a2e0 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.12) / linux-job (gh)
  RuntimeError: Command docker exec -t 2401509b497fd3508063f8d1ddd72946ad0d4a5102f1c0e961cf18b93248e87f /exec failed with exit code 1

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• Test CoreML Backend / test-coreml / test-backend-macos (coreml_static_int8, operators) / macos-job (gh)
  ##[error]The operation was canceled.
• Test XNNPACK Backend / test-xnnpack / test-backend-linux (xnnpack, models) / linux-job (gh)
  ##[error]The operation was canceled.

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• trunk / test-models-linux-aarch64 (resnet50, portable, linux.arm64.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  docker: Error response from daemon: unauthorized: authentication required.

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• trunk / test-llama-runner-mac (fp32, coreml) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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